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Tuesday, March 13, 2012

Sand Dollars ARE Sea Urchins. Please make a note of it!

(photo by km6xo)

So, last week, I was contacted by an intrepid member of the public who was quite interested in finding out more about sand dollars.. But apparently, the curiosity of this fellow had been stymied by the internet!GASP! Who would have thought that there could be a lot of questionable and apparently, conflicting information on the Internet!!! A lot of info on these animals may be somewhat basic to the well-travelled marine biologist, but maybe not so much to the curious or intrigued student or aspiring beach scholar! So, I thought today I would explain some basic sand dollars "stuff" and clarify some of the mystery.FIRST-some basic introductions........So, everyone is at least passingly familiar with sand dollars.

Those funny "dollar shaped shells" that one often finds walking along a beach down by the sea shore. How often have you seen this familiar sight?

photo by Jdigeronimo66

So, let's clarify this first- and foremost.

Sand dollars are the skeletons from ANIMALS.

Specifically, they are Echinoderms, which is the group that includes starfish, sea cucumbers, crinoids, and of course... sea urchins.

When sand dollars are alive, they are covered with a "fuzz" and look like this..

photo by Cheryl Moorehead

But following a little erosion and/or natural "cleaning" of the "fuzz" what you get is the internal skeleton:

photo by Cheryl Moorehead

And what you are seeing? is an INTERNAL skeleton (aka an endoskeleton) This even includes the spines (i.e., the "fuzz") because ultimately all of that (body skeleton+fuzz) is covered by a thin layer of skin or epidermis.The "fuzz" are actually the SPINES on a very strange looking sea urchin!Sand dollars are ANIMALS, specifically they are sea urchins! (Class Echinoidea) Sand dollars belong to a group known as the Clypeasteroida. There are some 75 genera of sand dollars, 29 living and 49 fossil (following Mooi 1989)with quite a few species.

Most sand dollars live in tropical shallow-water places (e.g., Africa, Singapore, Indonesia, the Bahamas, etc) but a few do live in cold to temperate waters (e.g., Dendraster excentricus on the west coast of North America) Sand dollars are NOT shells. Proper shells are deposited by organisms (such as mollusks) and are external to an animal's body.

And while we're discussing this, please note that sand dollars have ENDOSKELETONS rather than exoskeletons. That is to say, they are covered by skin and are considered "inside" the animal's body. A Sand dollar skeleton is known as a TEST.So What makes a sand dollar a sea urchin??Typically, we think of conventional sea urchins as looking kind of like this...

A big sized, ROUND ball covered by spines. These sea urchins often graze on algae an live out in the open on reefs or kelp beds. Often in large numbers.These have historically been referred to as "Regular" urchins. They have long, well-developed Spines and well-developed teeth as part of a elaborate jaw called Aristotle's Lantern. You can see all of these features in this video...Now, in CONTRAST....Sand dollars are members of a specialized sub-group of sea urchins that are often referred to as the "Irregular Urchins"These urchins differ quite a bit from the so-called "Regular" urchins because they show a suite of adaptations to living in sandy/muddy/ bottoms!In "irregular" sea urchins.. specifically sand dollars the following changes occur...1. The body (i.e., the test) changes from round and radial (in regulars) to flat and bilateral (in irregulars) like this...

(photo by Electropod)

to something more like this, which you'll note has both a left and a right side..

(photo by Avian-Cetacean Press)

2. Spines in "regular" urchins are usually elongate and pointed. But those in "irregular urchins" (esp. sand dollars) are short and specially modified to help in moving sediment..like so...

Here's what a single spine looks like under SEM close up. Not pointed but with a more blunt tip...

In a "regular" sea urchin, the Aristotle's Lantern or Jaw (seen here from the inside with the rest of the body removed) is used to feed on algae and its positioned as such..

Here's a neat video that shows the oral surface-and you can see the jaw's teeth in action emerging from the mouth

In contrast..here's the jaw (aka the Aristotle's Lantern) from a sand dollar. The top has been cut away and you can see it from the inside (mouth facing bottom). Its been modified into basically, a "crushing mill" for grinding up sand and so forth.

Here's a picture to show you more of what the "jaw" is like in other species.

The individual pieces of the jaw (aka the Aristotle's Lantern -which are often broken) is probably what you hear rattling around inside when you pick one up off the beach...

Plus, you'll often see it used in "inspirational" art and the famous "Legend of the Sand Dollar" postcards and related paraphenalia like this one...

The "doves of peace" are the broken fragments of the Aristotle's Lantern, i.e, the jaw the animal used to crush and grind sand. The "Star of Bethlehem" is a neatly dissected, complete jaw from the inside of the sand dollar.

5. The Body shape has changed as the body as evolved from that of a "regular" to an "irregular" urchin.

The above tree is used from Mooi, 1990 in Paleobiology!

The overall shape has seen a flattening out in the upper right part of the tree where we see sand dollars relative to their more globose relatives.

Excellent blog! I stumbled across it in my search to learn how sand dollars breathe. I have discovered they breathe by diffusion through tube feet. But i've seen differing answers online to whether they can breathe out of water when they're on the sand at low tide. Can you help me with this?

We were at beach this weekend and found 3 things that look kinda like sand dollars but they are not flat.... Is this a sand dollar or something else??? I have only ever seen the flat ones before now.... I told my kids I would help them find out what they are but have not found a picture on Internet any where.

In Wiltshire,England the fossils are collected on the Downs. 100 years ago my father was collecting "Shepherds Crowns", I still have them. Obviously it is a very long time since Wiltshire was under the sea.

Hello! I wanted to ask about how to take care of a sand dollar? I already know from your info that if has hair then it's alive. But I found it at the beach, and I don't know if it's alive or dead! If it's alive, how do I take care of it? I'm so confused!

Hi Kiara If you find a living sand dollar, I would just return it to the ocean and/or leave it where you found it. Sand dollars require food taken from the sand, and it generally requires a lot of special care to keep them in aquaria since the nutrients aren't really available from easily obtained sources.

Hi Chris. Very good scientific information complemented by interesting vídeos. I am portuguese but I live in Brazil where I met sand dollars for 1st time and I am in love, despiste being agronomic engineer. The ocean is fascinating. Thanks for sharing.

This is a great site - thank you so much for your time to present this information. It is just what I need to explain about sand dollars when I give him the 3 shells Ibrought home from the northern California coast. No other site came anywhere close to being this interesting and informative.

Thanks also had difficulty finding what I had found so now I guess I can stop calling it a sand dollar and refer to it as a sea biscuit. Very fragile and thicker than a dollar, thought it may be a relative of sea urchin. Have lived on beaches in Tasmania, Australia all my life but found my first sea biscuit at age fifty on a tidal river estuary and more recently closer to my home town on a very long sandy beach.

Great clarification on a beautiful creature I think I was calling a shell all my life-sorry! won't anymore, but what are the 3-4 rectangular cutouts in some sand dollar species from and for? Could you explain what the pretty design is from again?

i actuslly found a few urchin shells and sanddollars they were both dead but the sand dlollars broke when we got home but we were very careful with the urchin shells my mom actually ouund the urchin shells because she said they were floating around above the water

Thanks for your question. There is a pretty good fossil record for sand dollars-usually early Cenozoic (post-dinosaurs). But the largest I've seen any of them get is about the size of a medium sized dinner plate.

Hello, this was a fascinating article! I am visiting Nicaragua, where they call the sand dollars estrellas del mar, or stars of the sea. A google search for any such thing turns up starfish, so I'll consider these disk shaped creatures sand dollars.

As we walk along the beach, we see several that have recently washed ashore. If they are flipped upside down, we have been turning them over gently, at which time they scurry quickly under the sand. Many will say it is better to leave them alone, but I hate the thought of them scorching to death slowly just because of an unfortunate ride in a wave.

Please let me know if they are doomed to a certain untimely death if upside down on the beach, or if they are still all right if they are upside down. I don't want to mess with nature, but just as I would try to save a creature drowning or something, I would like to do a good deed for these lovely animals as well.

Greetings Becky,I appreciate your concerns. to be honest I'd need to see exactly what you are talking about to be sure but yes..if you are turning them back over and they are buyring themselves back into the sand then I'd approve. I don't know if they are "doomed" but taken at face value it sounds as if you are helping them. Since one probably cannot help ALL the animals that are like this, your behavior reflects more on you than on any direct widespread effect on the population..so I would wish you my best for your kind efforts.

Thank you so much for your encouraging response, Chris. We are certainly not able to walk the beach constantly looking for creatures to assist, but it is nice to hear that when we are able to help, we are likely doing that, and not harming the course of nature.

Thank you for the commentary on sand dollars. A friend recently brought me about 40 of the tiny variety that were part of more than one hundred they encountered on the beach. I've developed a special love for the tiny creatures and watch for these irregular urchins on the beach. I'd like to link a blog post about my preoccupation with them to this post so my readers can read more if they're interested.

Indeed they can. But I'm not sure if they are as abundant since they can occur in deeper water. Note that sometimes, finding sand dollars in shops does not guarantee they are local. Many are imported from America.

So long as you find the skeletons white and missing all of the spines they should be fine. Living urchins, including sea urchins, still have all their spines attached. Sand dollars would still have a "fuzzy" feeling to them rather than being smooth.

So, my question for the day is: how do they think? Do they have some sort of brain? Like, how do they know to dig in the sand to avoid predators? Do they recognize predators when they're nearby? If so, how? Smell? Sound?

I'm really intrigued by these guys, having found literally scores of them, all diff. sizes, this year on the shores of Coronado.

They don't have any well developed neural centers (i.e. no brain) and aren't known to be behaviorally complex. Most of their behavior is pretty much dictated by their basic functions-eat. spawn, etc. They can sense food in the water with strong chemoreception and presumably do the same with predators.

hi, Its not clear exactly where you are locatesd but you might be able to get some assistance from a paleontology or geology department at your local university or natural history museum. There are also several facebook groups which have citizen scientist/amateur fossil hobbyists.

Thanks Chris for a very informative blog which is still attracting comments 5 years on. I had never seen these sand dollars before until today. I found a heap of endoskeletons on horseshoe bay beach on magnetic island on the great barrier reef, off the coast from Townsville,QLD, Australia.These ones are 'penta' shaped and seem to have an a mix of regular and irregular features... which I guess makes then irregular... thnx

I assume you mean by humans? And so I would say mostly not. Sand dollar body structure is primarily skeleton and spines. I suppose its possible that someone might have developed a taste for the uni (gonads) the way other sea urchins are eaten but that has yet to be demonstrated as anything more than a chance irregularity.

About Me

I pursue starfish related adventure around the world with a critical eye and an appreciation for weirdness.
Support has been courtesy of the National Science Foundation but the views and opinions presented herein are mine and do not reflect the opinions of them or any affiliated institutions.
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